Semiconductor laser module

ABSTRACT

A problem of the present invention is to provide a semiconductor laser module for making precise temperature control possible.  
     A semiconductor laser module  1  according to the present invention has a Peltier element  4,  a thermistor  5  for detecting the temperature of the Peltier element  4,  and a differential amplification circuit  6  for generating a control signal according to the detection signal from the thermistor  5  so that the surface temperature of the Peltier element  4  conforms to a set temperature. A parallel compensating circuit  12  for generating a compensating signal for compensating for a delay in temperature variation in the Peltier element  4  with respect to the input of current signal to the Peltier element  4  branches and is connected to the output portion of the differential amplification circuit  6.  The compensating signal is sent to an addition circuit  9  and detection correction signal obtained by adding the detection signal of the thermistor  5  and the compensating signal is generated. The detection correction signal is sent to the differential amplification circuit  6  and a control signal corresponding to the difference between the detection correction signal and a signal corresponding to set temperature is generated.

TECHNICAL FIELD

[0001] The present invention relates to a semiconductor laser modulehaving the function of controlling the temperature of a semiconductorlaser.

BACKGROUND ART

[0002] As a semiconductor laser module having the function ofcontrolling the temperature of a semiconductor laser, there is a knownunit described in FLD5F14CN-E19 to E58 of FUJITSU SEMICONDUCTOR DEVICEDATA SHEET (DS02-12601-2). The automatic temperature control circuitdescribed in the DATA SHEET has a Peltier element and a thermistorcontained in a laser diode and an analog integrator and used fortemperature control by proportion integration (P. I) control so that theactual temperature conforms to a set temperature.

[0003] Absolute precision and absolute stability of wavelength arerequired in the wavelength division multiplexing (WDM) communicationsystem. Although distribution feedback (DFB) type semiconductor lasersare employed in such a WDM communication system, the temperature of thelaser at the time of oscillation needs controlling precisely because theoscillation wavelength of the DFB type laser is easily affected by thetemperature of the laser. The oscillation wavelength between adjoiningchannels tends to be set at 0.8 nm or less in the WDM communicationsystem in recent years. The required wavelength precision of the laserfor use in the system like this is 0.01 nm. In order to attain theserequirements, it is needed to control the temperature of the laser atthe time of oscillation within 0.1° C.

[0004] In the above-described prior-art automatic temperature controlcircuit, a steady state may be brought about in a state that the actualtemperature deviates from a set temperature under the influence ofthermal resistance and the like. In order to increase the precision ofthe temperature control by reducing the deviation between the actualtemperature and the set temperature, the loop gain of a control systemneeds increasing. In the automatic temperature control circuit having aclosed loop using the Peltier element, however, though it is possible tofreely set a temperature in the case of PI control through digitalprocessing, there occurs a great delay in transmission besides theprimary delay between the Peltier element and the thermistor in the caseof analog processing as the system is generally such that the responseof the temperature is extremely slow in comparison with the response ofthe electronic circuit. For this reason, oscillation is likely to arisewhen the loop gain of the control system is increased too much.Therefore, the loop gain cannot be increased immoderately and this makesit difficult to raise the precision of the temperature control.

[0005] An object of the present invention is to provide a semiconductorlaser module for making precise temperature control possible.

DISCLOSURE OF THE INVENTION

[0006] As a result of studies made earnestly on the foregoing problems,the present inventors have found that when input to an electronicrefrigeration element is varied stepwise in the steady state incontrolling the temperature of a semiconductor laser module using theelectronic refrigeration element (a Peltier element) and a temperaturedetection element, a delay a rises from a point of time when the inputthereto is varied until the output of the temperature detection elementbegins to actually vary and the delay greatly contributes to theoscillation as well as the primary delay. An object of the presentinvention is to cancel the delay.

[0007] More specifically, a semiconductor laser module according to theinvention comprises: a semiconductor laser, laser temperature controlmeans having an electronic refrigeration portion and a temperaturedetection portion for detecting the temperature of the electronicrefrigeration portion and used for controlling the temperature of thesemiconductor laser, and control signal generating means for inputting adetection signal of the temperature detection portion, generating acontrol signal for conforming the temperature of the electronicrefrigeration portion with a set temperature and sending the controlsignal to the electric refrigeration portion, wherein the control signalgenerating means generates a compensating signal for compensating for adelay of temperature variation of the electronic refrigeration portionwith respect to an input of the control signal to the electronicrefrigeration portion, and generates a detection correction signalprovided by correcting the detection signal by the compensating signalso as to generate the control signal according to the detectioncorrection signal and a signal corresponding to the set temperature.

[0008] The provision of the control signal generating means makes itpossible to obtain the detection correction signal which reduce thedelay of actual temperature variation with respect to the settemperature variation as the detection signal is compensated by thecompensating signal even though the actual temperature variation withrespect to the set temperature variation is detected by the temperaturedetection portion with some delay caused by a delay in response betweenthe electronic refrigeration portion and the temperature detectionportion. The control signal is generated according to the detectioncorrection signal and the signal corresponding to the set temperatureand then inputted to the electric refrigeration portion. Therefore,application of energy more than necessary is prevented from being givento the electronic refrigeration portion immediately after the settemperature in the steady state is changed. As the loop gain of thecontrol system can be increased thereby, the actual temperature of theelectronic refrigeration portion is prevented from deviating from theset temperature in the steady state. Therefore, the electronicrefrigeration portion can be put under precise temperature control.

[0009] Preferably, the control signal generating means has a filtercircuit for inputting the control signal and generating the compensatingsignal, and an addition circuit for adding the detection signal and thecompensating signal to generate the detection correction signal. Thus,means for generating the compensating signal and means for generatingthe detection correction signal are attainable with a simple circuitconfiguration. In the feedback loop in this case, the whole system ismade controllable in an optimum mode by setting the values of resistorsand capacitors constituting the filter circuit so that they have afilter time constant and a feedback quantity as desired.

[0010] In this case, preferably the control signal generating means hasa first operational amplifier having one input portion to which thedetection signal is input and the other input portion to which thecontrol signal is input via a first capacitor, and a second capacitorand a resistor connected in parallel between the other input portion andan output portion of the first operational amplifier, and the firstoperational amplifier, the first capacitor, the second capacitor, andthe resistor make up the filter circuit and the addition circuit. Inthis case, the number of operational amplifiers used with the filtercircuit and the addition circuit may be one, so that the circuitconfiguration of the module is simplified.

[0011] At this time, preferably the control signal generating meansfurther has a second operational amplifier having an output portionconnected to the other input portion of the first operational amplifiervia the first capacitor, and a third capacitor connected between theoutput portion and one input portion of the second operationalamplifier, and the third capacitor forms a part of the filter circuit.In this case, the number of capacitors serving the function as alow-pass filter becomes two, so that two small-sized capacitors can beappropriately combined to form a compensating signal having any desiredtime constant.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a configuration diagram to show a first embodiment of asemiconductor laser module according to the invention.

[0013]FIG. 2 is a diagram to show a specific circuit configuration ofthe semiconductor laser module shown in FIG. 1.

[0014]FIG. 3 is operation waveform charts of the semiconductor lasermodule shown in FIG. 1; (a) is an operation waveform chart of settemperature; (b) is an operation waveform chart of the output value of abias circuit; (c) is an operation waveform chart of the output value ofa parallel compensating circuit; and (d) is an operation waveform chartof the output value of an addition circuit.

[0015]FIG. 4 is a diagram to show the circuit configuration of a generalsemiconductor laser module in a prior art.

[0016]FIG. 5 shows comparison of current signals applied to Peltierelements; (a) is a drawing to show the current signal of thesemiconductor laser module shown in FIG. 4 and (b) is a drawing to showthe current signal of the semiconductor laser module shown in FIG. 1.

[0017]FIG. 6 is a diagram to show the circuit configuration of a secondembodiment of a semiconductor laser module according to the invention.

[0018] Reference numerals in the figures areas follows: Numeral 1denotes a semiconductor laser module, numeral 2 denotes a semiconductorlaser, numeral 3 denotes a laser temperature control element portion(laser temperature control means), numeral 4 denotes a Peltier element(electric refrigeration portion), numeral 5 denotes a thermistor(temperature detection portion), numeral 6 denotes a differentialamplification circuit, numeral 7 denotes a bias circuit, numeral 9denotes an addition circuit, numeral 12 denotes a parallel compensatingcircuit, numeral 21 denotes a high-pass filter circuit portion (filtercircuit), numeral 22 denotes a low-pass filter circuit portion (filtercircuit), numeral 46 denotes an operational amplifier (secondoperational amplifier), numeral 70 denotes a semiconductor laser module,numeral 71 denotes a differential amplification circuit, numeral 72denotes a capacitor (third capacitor), numeral 73 denotes an operationalamplifier (first operational amplifier), numeral 74 denotes a capacitor(first capacitor), numeral 76 denotes a resistor, and numeral 77 denotesa capacitor (second capacitor).

BEST MODE FOR CARRYING OUT THE INVENTION

[0019] A description of a semiconductor laser module as the preferredembodiments of the present invention will now be given by reference tothe drawings.

[0020]FIG. 1 is a configuration diagram to show a first embodiment of asemiconductor laser module according to the invention. As shown in thefigure, a semiconductor laser module 1 according to this embodiment isprovided with a semiconductor laser (laser diode) 2 and a lasertemperature control element portion 3 for controlling the temperature ofthe semiconductor laser 2. The laser temperature control element portion3 has a Peltier element 4 as an electronic refrigeration element and athermistor 5 as a temperature detection element for detecting thesurface temperature of the Peltier element 4. In this case, thesemiconductor laser 2 and the laser temperature control element portion3 are in the form of, for example, one packaged integrated circuit.

[0021] The semiconductor laser module 1 has a differential amplificationcircuit 6 and a set signal (voltage) corresponding to a set temperature(target temperature) of the surface of the Peltier element 4 is appliedto the non-inverting-(+)-side input portion of the differentialamplification circuit 6. The thermistor 5 is connected to theinverting-(−)-side input portion of the differential amplificationcircuit 6 via an addition circuit 9 and a bias circuit 7. Thenon-inverting-side output portion and the inverting-side output portionsof the differential amplification circuit 6 are connected via a driver11 to the two metal electrodes (not shown) of the Peltier element 4. Thedifferential amplification circuit 6 like this compares the inputvoltage of non-inverting-side input portion with the input voltage ofinverting-side input portion and amplifies a signal corresponding to thedifference between the input voltages with a predetermined amplifyingdegree (gain) to generate the control signal (voltage). Then the outputvoltage of the differential amplification circuit 6 is converted by thedriver 11 into a current signal and introduced into the Peltier element4.

[0022] The Peltier element 4, the thermistor 5, the bias circuit 7, theaddition circuit 9, the differential amplification circuit 6 and thedriver 11 constitute a feedback control system as one closed loop inorder to control the temperature such that the surface temperature ofthe Peltier element 4 conforms to the set temperature. In this case, thesurface temperature of the Peltier element 4 lowers as the current valueinput to the Peltier element 4 increases, whereas the surfacetemperature of the Peltier element 4 rises as the current value input tothe Peltier element 4 decreases.

[0023] A parallel compensating circuit 12 branches and is connected tothe inverting-side output portion of the differential amplificationcircuit 6. The parallel compensating circuit 12 generates a compensatingsignal for compensating for a delay of temperature variation of thePeltier element 4 with respect to input of a current signal to thePeltier element 4. The compensating signal is a signal with directcurrent (DC) component cut (see FIG. 3(c)). The output signal of theparallel compensating circuit 12 is input to the addition circuit 9,forming a local loop. The addition circuit 9 adds the output signal ofthe bias circuit 7 corresponding to the detection signal of thethermistor 5 and the output signal of the parallel compensating circuit12 to generate a detection correction signal (see FIG. 3(d)).

[0024] Here, the differential amplification circuit 6, the bias circuit7, the parallel compensating circuit 12, and the addition circuit 9 makeup control signal generating means for generating a control signalsupplied through the driver 11 to the Peltier element 4.

[0025]FIG. 2 shows a specific circuit configuration of the semiconductorlaser module 1 described above. In the figure, the parallel compensatingcircuit 12 is provided with a high-pass filter (HPF) circuit portion 21and a low-pass filter (LPF) circuit portion 22.

[0026] The HPF circuit portion 21 has an operational amplifier 23, and areference voltage V_(r1) is input to an in-phase input portion of theoperational amplifier 23. A negative-phase input portion of theoperational amplifier 23 is connected to the inverting-side outputportion of the differential amplification circuit 6 via a capacitor 24.The negative-phase input portion and output portion of the operationalamplifier 23 are connected via a resistor 25, and a capacitor 26 isconnected in parallel to the resistor 25.

[0027] The LPF circuit portion 22 has an operational amplifier 27, and areference voltage V_(r2) is input to an in-phase input portion of theoperational amplifier 27. A negative-phase input portion of theoperational amplifier 27 is connected to the output portion of theoperational amplifier 23 via a resistor 28. The negative-phase inputportion and output portion of the operational amplifier 27 are connectedvia a resistor 29, and a capacitor 30 is connected in parallel to theresistor 29.

[0028] The addition circuit 9 has an operational amplifier 31, and areference voltage V_(r3) is input to an in-phase input portion of theoperational amplifier 31. A negative-phase input portion of theoperational amplifier 31 is connected via resistors 32 and 33 to theoutput portion of the operational amplifier forming the bias circuit 7and the output portion of the operational amplifier 27. Thenegative-phase input portion and output portion of the operationalamplifier 31 are connected via a resistor 34.

[0029] The differential amplification circuit 6 has operationalamplifiers 35 and 36. An in-phase input portion of the operationalamplifier 35 is connected to the output portion of the operationalamplifier 31, and a negative-phase input portion of the operationalamplifier 35 is grounded via a resistor 37. The negative-phase inputportion and output portion of the operational amplifier 35 are connectedvia a resistor 38. A set signal is input to an in-phase input portion ofthe operational amplifier 36, and a negative-phase input portion of theoperational amplifier 36 is grounded via a resistor 39. Thenegative-phase input portion and output portion of the operationalamplifier 36 are connected via a resistor 40. The output portion of theoperational amplifier 35 is connected to a negative-phase input portionof an operational amplifier 42 via a resistor 41, and the output portionof the operational amplifier 36 is connected to an in-phase inputportion of the operational amplifier 42 via a resistor 43. Thenegative-phase input portion and output portion of the operationalamplifier 42 are connected via a resistor 44. The output portion of theoperational amplifier 42 is connected to a negative-phase input portionof an operational amplifier 46 via a resistor 45, and a referencevoltage V_(r4) is input to an in-phase input portion of the operationalamplifier 46 via a resistor 47. The negative-phase input portion andoutput portion of the operational amplifier 46 are connected via aresistor 48. The output portion of the operational amplifier 46 isconnected to a negative-phase input portion of an operational amplifier50 via a resistor 49, and the reference voltage V_(r4) is input to anin-phase input portion of the operational amplifier 50 via a resistor51. The negative-phase input portion and output portion of theoperational amplifier 50 are connected via a resistor 52. The outputportions of the operational amplifiers 46 and 50 are connected to thedriver 11.

[0030] In the semiconductor laser module 1 thus arranged, when the settemperature T_(o) of the surface of the Peltier element 4 is raisedstepwise in a steady state (see FIG. 3(a)), the actual surfacetemperature of the Peltier element 4 is controlled to go closer to theset temperature T_(o).

[0031] The closed loop in connection with the above-describedtemperature control is characterized in that the response of thetemperature is extremely slower than the response of the electroniccircuit; that is, the system has an extremely large time constant withinthe closed loop (between the Peltier element 4 and the thermistor 5).Consequently, even though the current value supplied to the Peltierelement 4 is varied for a short period of time, the surface temperatureof the Peltier element 4 is unable to follow the variation intemperature immediately but varied with a predetermined time delay andthis is detected by the thermistor 5. Therefore, the output signal ofthe bias circuit 7 corresponding to the detection signal of thethermistor 5 becomes a signal as shown in FIG. 3(b).

[0032] At this time, the output signal from the inverting-side outputportion of the differential amplification circuit 6 is sent to theparallel compensating circuit 12, which then generates a compensatingsignal for compensating for a delay of temperature variation of thePeltier element 4 with respect to input of a current signal to thePeltier element 4. The compensating signal is such a signal that asshown in FIG. 3(c) its level rises from a point of time that the settemperature T_(o) of the Peltier element 4 has changed and thengradually falls after the predetermined time passes and that finally itsDC component is reduced to zero. A time constant of the compensatingsignal can be adjusted appropriately by setting the values of resistorand capacitor of the parallel compensating circuit 12 as desired.

[0033] Such a compensating signal is sent to the addition circuit 9,which then adds the detection signal (output value of the bias circuit7) and the compensating signal to generate a detection correctionsignal. The detection correction signal is as shown in FIG. 3(d) almostfree from any delay with respect to the variation of the set temperatureT_(o); in other words, it is a signal that the primary delay is causedimmediately after the set temperature T_(o) is changed.

[0034] The detection correction signal is sent to the differentialamplification circuit 6, and a control signal corresponding to thedifference between the detection correction signal and the set signalcorresponding to the set temperature T_(o) is generated. This controlsignal is converted by the driver 11 to a current signal before beinginput to the Peltier element 4. Accordingly, the surface temperature ofthe Peltier element 4 converges to set temperature T_(o).

[0035] Here, as a comparison example, FIG. 4 shows the circuitconfiguration of a general semiconductor laser module in a prior art. Inthe figure, a semiconductor laser module 100 differs from thesemiconductor laser module 1 in that it does not have the bias circuit7, the addition circuit 9, and the parallel compensating circuit 12described above, namely, no local loop exists. In such a configuration,a detection signal of a thermistor 5 equivalent to the signal shown inFIG. 3(b) is input to an inverting-side input portion of a differentialamplification circuit 6 (in-phase input portion of operational amplifier35) as it is. A current signal responsive to the difference between thedetection signal and the set signal corresponding to the set temperatureT_(o) is supplied to the Peltier element 4.

[0036] In this case, the current value applied to the Peltier element 4is expressed by a portion A₁ with oblique lines surrounded with the settemperature T_(p) and the detection signal T in FIG. 5(a). At this time,though the detection signal T rises after the passage of a predeterminedtime B from a point of time of the change of the set temperature T_(p),the application of the current to the Peltier element 4 is continuedeven for the delayed period B. As the detection signal T remainsunchanged during this period, a relatively large current value is to beapplied to the Peltier element 4. The fact that great energy of thecurrent signal applied to the Peltier element 4 significantlycontributes to an excessive fall (undershoot) or an excessive rise(overshoot) in the temperature control of the Peltier element 4 has beenascertained by experiment.

[0037] That is, even though the current value supplied to the Peltierelement 4 is reduced when the surface temperature of the Peltier element4 reaches the set temperature T_(o), the surface temperature of thePeltier element 4 is unable to follow the variation in temperatureimmediately but begins to follow the variation in temperature after anundershoot or an overshoot. Then, the undershoot or overshoot isrepeated. In case where the setting of the time constant of anelectronic circuit is appropriate then, the undershoot or the overshootis gradually decreased and the surface temperature of the Peltierelement 4 is going closer to the set temperature T_(o) and in case wherethe setting of the time constant is inappropriate, an oscillationphenomenon occurs.

[0038] In order to suppress such oscillation by decreasing theundershoot and overshoot of the temperature, it is only needed to reducethe gain of the differential amplification circuit 6, for example.However, the reduction of the gain of the differential amplificationcircuit 6 is not preferred because the deviation of the actualtemperature from the set temperature T_(o) may induce the steady statewith the establishment of the following equation (A).

T _(M) =T _(o) −αq/A  (A)

[0039] (where T_(M): equilibrium temperature, T_(o): set temperature, α:functions of thermal capacity and thermal resistance, q: outflow thermalquantity per second, and A: gain of differential amplification circuit)

[0040] In contrast, in the embodiment, since the parallel compensatingcircuit 12 and the addition circuit 9 are provided and the output signalof the parallel compensating circuit 12 is fed back into the input ofthe differential amplification circuit 6 via the addition circuit 9, thecurrent value applied to the Peltier element 4 at this time is expressedby a portion A with oblique lines surrounded with the set temperatureT_(p) and a detection correction signal T_(S) in FIG. 5(b). As thedetection correction signal T_(S) is set so that the timing of outputvariation with respect to the variation of set temperature T_(p) isaccelerated as described above, the current value (energy) applied tothe Peltier element 4 during the delayed period B of variation of thedetection signal with respect to the variation of the set temperatureT_(p) is decreased in comparison with the case of FIG. 5(a).

[0041] In feedback control of the surface temperature of the Peltierelement 4, since the gain of the differential amplification circuit 6can thus be increased without occurrence of an oscillation phenomenon,the surface temperature of the Peltier element 4 is prevented from beingin the steady state while the surface temperature thereof is deviatedfrom the set temperature. The surface temperature of the Peltier element4 can therefore be controlled with high precision and stability.Consequently, wavelength precision necessary for the wavelength divisionmultiplexing (WDM) communication system can be secured.

[0042] In the case where the delay time B of the detection signal of thethermistor 5 is 100 mS, it has been clarified by experiment that thesystem as a whole is made controllable quickly and stably by setting thevalues of the resistors and the capacitors constituting the parallelcompensating circuit 12 so that the time constant of the parallelcompensating circuit 12 ranges from 200 to 300 ms and the feedbackquantity in a feedback loop ranges approximately from 20 to 30%.

[0043] Moreover, as it is possible to compensate for a delay intemperature variation in the Peltier element 4 with respect to the inputof current signal to the Peltier element 4 by means of a simple circuitcomprising the parallel compensating circuit 12 and the addition circuit9, this arrangement is advantageous in view of cost reduction.

[0044]FIG. 6 is a circuit configuration diagram to show a secondembodiment of a semiconductor laser module according to the invention.Parts identical with those in the first embodiment are denoted by thesame reference numerals in the figure and will not be discussed again.

[0045] In the figure, a semiconductor laser module 70 of the embodimenthas a differential amplification circuit 71 comprising a capacitor 72connected in parallel to the resistor 48 in the differentialamplification circuit 6 in the first embodiment.

[0046] The semiconductor laser module 70 has an operational amplifier73. An in-phase input portion of the operational amplifier 73 isconnected to a thermistor 5, and a negative-phase input portion of theoperational amplifier 73 is connected to an output portion of anoperational amplifier 46 of the differential amplification circuit 71via a capacitor 74 and a resistor 75. The negative-phase input portionand output portion of the operational amplifier 73 are connected via aresistor 76, and a capacitor 77 is connected in parallel to the resistor76. The output portion of the operational amplifier 73 is connected toan in-phase input portion of an operational amplifier 35 of thedifferential amplification circuit 71. In such a circuit configuration,the gain of the operational amplifier 73 can be changed appropriately bysetting the values of the resistors 75 and 76 as desired.

[0047] In the semiconductor laser module 70, the operational amplifier73 has a function as the addition circuit 9 in the first embodiment.That is, a detection signal of the thermistor 5 is input to the in-phaseinput portion of the operational amplifier 73, and a signal output froman inverting-side output portion of the differential amplificationcircuit 71 (the output portion of the operational amplifier 46) is inputto the negative-phase input portion of the operational amplifier 73 andconsequently the detection signal of the thermistor 5 and the outputsignal of the operational amplifier 46 are added. The operationalamplifier 73, the resistor 76, and the capacitors 72, 74, and 77 have afunction as the parallel compensating circuit 12 in the firstembodiment. Specifically, the capacitor 74 serves as an HPF and thecapacitors 72 and 77 serve as an LPF. Therefore, the semiconductor lasermodule 70 operates in a similar manner to that of the semiconductorlaser module 1 of the first embodiment.

[0048] In the described embodiment, the number of the operationalamplifiers used is drastically decreased as compared with the firstembodiment, so that the manufacturing costs can be reduced. Since thecircuit configuration of the semiconductor laser module is downsized,the module itself can be miniaturized. Small-sized capacitors can beused in combination as the capacitors 72 and 77 serving as the LPF toprovide a compensating signal having an optimum time constant.Therefore, the module itself can also be miniaturized in this point.Further, the capacitor 72 is connected to the operational amplifier 48at the output stage of the differential amplification circuit 71, sothat it is made possible to suppress oscillation in a high-frequencyband.

[0049] The invention is not limited to the above-described embodimentsthereof. Although according to this embodiment of the invention, thecompensating signal with the DC component being cut is formed with acircuit inclusive of a simple high-pass filter, for example, it mayotherwise be formed with a one-shot multivibrator which generates pulsesignal with predetermined time by the predetermined timing signal andthe like.

[0050] Although according to this embodiment of the invention, thesignal with the DC component being cut is finally generated as thecompensating signal, an arrangement for generating a compensating signalhaving such a DC component may be adopted. This arrangement issatisfactorily usable by shifting the gain of the differentialamplification circuit and the set temperature so that temperaturecontrol is adequately less affected by the DC component, for example. Incase where the compensating signal has the DC component, theabove-described equation (A) is expressed by the following equation (B).In this case, however, a DC level B₀ may be set so as to cancel thedeviation of an equilibrium temperature T_(H) from the set temperatureT_(o) and a higher gain of the differential amplification circuitbecomes unnecessary at this time.

T _(H) =T _(o) −αq(1+B ₀ ×A)/A  (B)

[0051] (where T_(H): equilibrium temperature, T_(o): set temperature, α:functions of thermal capacity and thermal resistance, q: outflow thermalquantity per second, B₀: DC level, and A: gain of differentialamplification circuit).

[0052] While the invention has been particularly shown and describedwith reference to specific embodiments thereof, it will be understood bythose skilled in the art that various changes and corrections can bemade therein without departing from the spirit and scope of theinvention.

[0053] The application is based on Japanese patent application(2000-371618) filed on Dec. 6, 2000 and Japanese patent application(2001-368927) filed on Dec. 3, 2001, and the contents are taken thereinas reference.

[0054] Industrial Applicability

[0055] According to the invention, the compensating signal forcompensating for a delay in temperature variation in the electronicrefrigeration portion with respect to the input of control signal to theelectronic refrigeration portion is generated, the detection correctionsignal provided by correcting the detection signal by the compensatingsignal is generated, and the control signal is generated according tothe detection correction signal and the signal corresponding to the settemperature, whereby the precision of the temperature control in theelectronic refrigeration portion can be improved. As the temperature ofthe semiconductor laser at the time of oscillation is controlled withhigh precision, wavelength precision necessary for the wavelengthdivision multiplexing (WDM) communication system can thus be securedsatisfactorily.

1. A semiconductor laser module comprising a semiconductor laser, lasertemperature control means having an electronic refrigeration portion anda temperature detection portion for detecting a temperature of theelectronic refrigeration portion, said laser temperature control meansfor performing temperature control of said semiconductor laser, andcontrol signal generating means for inputting a detection signal of thetemperature detection portion, generating a control signal forconforming the temperature of the electronic refrigeration portion witha set temperature, and sending the control signal to the electronicrefrigeration portion, characterized in that said control signalgenerating means is means for generating a compensating signal forcompensating for a delay of a temperature variation of theelectronic-refrigeration portion with respect to an input of the controlsignal to the electronic refrigeration portion, generating a detectioncorrection signal provided by correcting the detection signal by thecompensating signal, and generating the control signal based on thedetection correction signal and a signal corresponding to the settemperature.
 2. The semiconductor laser module as claimed in claim 1wherein said control signal generating means has a filter circuit forinputting the control signal and generating the compensating signal andan addition circuit for adding the detection signal and the compensatingsignal to generate the detection correction signal.
 3. The semiconductorlaser module as claimed in claim 2 wherein said control signalgenerating means has a first operational amplifier having one inputportion to which the detection signal is input and the other inputportion to which the control signal is input via a first capacitor, anda second capacitor and a resistor connected in parallel between theother input portion and an output portion of the first operationalamplifier and wherein the first operational amplifier, the firstcapacitor, the second capacitor, and the resistor make up the filtercircuit and the addition circuit.
 4. The semiconductor laser module asclaimed in claim 3 wherein said control signal generating means furtherhas a second operational amplifier having an output portion connected tothe other input portion of the first operational amplifier via the firstcapacitor, and a third capacitor connected between the output portionand one input portion of the second operational amplifier and whereinthe third capacitor forms a part of the filter circuit.